Most of ultraviolet ray normally received by a human body comes from sunlight. The ultraviolet ray included in sunlight is divided to a long wavelength ultraviolet ray having a wavelength from 400 nm to 320 nm (UV-A), a medium wavelength ultraviolet ray having a wavelength from 320 nm to 290 nm (UV-B), and a short wavelength ultraviolet ray having a wavelength of not more than 290 nm in a field of skin science. Of these, the ultraviolet rays having a wavelength of not more than 290 nm are absorbed by the ozonosphere and does not reach the surface of the earth.
The ultraviolet rays reaching the surface of the earth give various influences to human skin. Of the ultraviolet rays reaching the surface of the earth, the UV-A makes color of human skin brown, reduces the elasticity of skin, and promote generation of wrinkles, thus causing rapid aging of human skin. Also the ultraviolet rays promote start of the erythema reaction, or further stimulates this reaction in certain types of patients, and furthermore sometimes may cause photo-toxicity or photo allergy reactions. On the other hand, also UV-B forms erythema or blister on human skin, and causes aggravation of melanin formation or other changes such as deposition of pigments.
For the reasons as described above, in order to prevent aging of human skin and generation or increase of wrinkles and flecks thereon, it is very important to protect human skin from the ultraviolet rays, and various types of ultraviolet absorbent have been developed for that purpose.
The existing ultraviolet absorbents used for practical application include a PABA derivative, a cinnamic acid derivative, a salicylic acid derivative, a benzophenone derivative, a urocanine derivative, a campher derivative, and heterocyclic derivatives.
These types of ultraviolet absorbent are generally mixed in external preparations for skin such as cosmetics, or quasidrugs, and a low molecular weight dimethyl-siloxane base has been widely used as a base for the external preparation for skin.
Namely, as anti-suntan agent are most frequently used during summer, oily bases have been used because of their anti-sweat property as well as their high water resisting property, and as a result, most of materials which have been used for ultraviolet absorbents are oil soluble.
Recently, however, social attention has been focused on effects of ultraviolet rays on human skin in our daily life, so that anti-suntan materials are desired for ordinary skin care. For that purpose, development of materials which are water soluble and at the same time absorb ultraviolet rays has been strongly demanded, because such materials can mixed in aqueous external preparation such as skin lotion in a large volume, and also because a large volume of ultraviolet absorbent is required to be mixed in the entire system to realize a external preparations having the high capability to absorb ultraviolet rays, which makes it necessary to mix a ultraviolet absorbent not only in oil phase but also in aqueous phase.
The prior types of ultraviolet absorbent, however, are oil soluble in most cases, and their solubility in water is very low, and their applications have been limited to a relatively narrow area. As a water soluble ultraviolet absorbent, only 2-hydroxy-4-methoxy-5-sulfoxonium benzophenone sodium salt has been known, and as this is a salt, it effects pH of the formula system.